HVAC ducts made of plastic are known in the art. For example, U.S. Pat. No. 8,667,995 discloses an HVAC duct for use in an HVAC system. The duct includes a plastic outer tube having a fully continuous external perimeter, a plastic inner tube capable of conveying forced air and a plurality of plastic support elements spanning between the outer tube and the inner tube. The support elements extend along the inner tube and the outer tube lengthwise and define a plurality of channels between the outer tube and the inner tube. A plurality of thermally insulating layers is contained within the channels.
A shortcoming of plastic HVAC ducts is their inability to withstand direct exposure to flames. Because of this deficiency, plastic HVAC ducts often fail to meet building codes that incorporate UL standards including, for example, UL 181 (Flame Penetration Test), which sets forth requirements for factory-made air ducts and air connectors. An HVAC plastic duct that fails to meet this standard is not suitable for installation in those jurisdictions that have adopted UL standards. Accordingly, there is a need for an HVAC plastic duct that is flame resistant.